1. Field of the Invention
The present invention relates to a projection apparatus using a light source for emitting coherent light beams and a hologram recording medium.
2. Description of the Related Art
It is important for optical projection apparatuses (projectors) to uniformly illuminate a screen with light radiated from a light source. As methods for realizing uniform illumination of a screen, a 3-panel liquid crystal system using fly-eye lenses, an integrator-rod system for DMD (Digital Micromirror Device), etc. have been proposed.
However, in these systems, there is a problem in that restrictions on optical arrangements determined according to the focal distances of lenses of these systems are very severe in order to obtain rectangular illumination in accordance with the shape of a liquid crystal display, because design freedom is low and a large number of parts are necessary.
Moreover, the integrator-rod system requires a clean rod end face because an image due to dust, a scratch, etc., on the rod end face is formed on a micro display.
Generally, in optical projectors, a spatial light modulator such as a liquid crystal display is illuminated using an illumination device having a white light source such as a high pressure mercury lamp to obtain a modulated image, and the obtained modulated image is magnified and projected onto a screen through lenses.
However, high intensity discharge lamps such as high pressure mercury lamps have a relatively short lifecycle and when they are used for optical projectors or the like, they need to be frequently replaced. In order to cope with such problems, a system using a coherent light source such as a laser has also been proposed. For example, semiconductor lasers which have been widely used in industries have a very long lifecycle in comparison with high intensity discharge lamps such as high pressure mercury lamps. In addition, since semiconductor lasers are capable of generating light of a single wavelength, a color separation apparatus such as a dichroic mirror is unnecessary, so that there is an advantage in that the entire system becomes smaller.
However, in the system using a coherent light source of laser beams or the like, there is a new problem in that speckle is generated. Speckle is a spotted pattern which is formed when a coherent light beam such as a laser beam is emitted to a scattering plane. If speckle is generated on a screen, they are observed as spotted luminance unevenness, i.e. brightness unevenness, thus becoming a factor of having physiologically adverse affect on an observer. It is considered that the reason why speckle is generated in the case of using coherent light beams is that coherent light beams reflected from respective portions of a scattering and reflecting plane such as a screen have very high coherency so that coherent light beams interfere with one another to generate speckles. For example, a theoretical review of the generation of speckles is made in detail in Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.
As discussed above, there is a problem about speckle which is coherent nature of laser light source. Therefore, techniques for suppressing the generation of speckle have been proposed. For example, Japanese Patent Laid-Open No. 6-208089 discloses a technique in which a laser beam is emitted to a scattering plate, scattered light beams obtained therefrom are guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, thus reducing speckles.